Aluminum alkyl cocatalysts

The Lewis acidity and reactivity of these alkyl aluminum cocatalysts and activators with Lewis basic polar monomers such as acrylates make them impractical components in the copolymerization of ethylene with acrylates. To address this shortcoming, Brookhart et al. developed well-defined cationic species such as that shown in Fig. 2, in which the counterion (not illustrated) was the now-ubiquitous fluorinated arylborate family [34] such as tetrakis(pentaflurophenyl)borate. At very low methyl acrylate levels the nickel catalysts gave linear copolymers but with near-zero levels of acrylate incorporation. 

It is speculated that the use of heterogeneous or partially heterogeneous Nd catalyst systems results in gel formation. Due to this reason, Nd-systems which are soluble in hydrocarbon solvents are preferred today, especially in large-scale operations. The soluble catalysts are usually based on ternary systems which consist of Nd salts with anions bearing long-chain aliphatic groups, an alkyl aluminum cocatalyst and a halide donor. 

Another feature of Nd-catalyzed BD polymerization which is reported by various authors is the reduction of cis- 1,4-content by the addition of donor compounds. The respective increase of trans- 1,4-moieties and decrease of cis- 1,4-moieties is also observed when the amount of the alkyl aluminum cocatalyst is increased. This feature does not depend on the nature of the Nd component and has for example been comparatively studied for three different catalyst systems (1) NdV/DIBAH/EASC, (2) neodymium neopen-... 

Figure 3. Conversion of isoprene to solid polymer as a function of Al/Ti molar ratio and groups in alkyl-aluminum cocatalyst polymerization time—22 hours.

Polymerization reaction is initiated by the active sites located inside the catalyst pores where ethylene and the alkyl-aluminum cocatalyst penetrate. As the polyolefin chains grow, stress develops inside the catalyst pores and small particles are... 

In alkyl aluminum chlorides of the type RxAlyClz two different chemical moieties which cause alkylation as well as chlorination are present in one molecule. Therefore, RAAL,Clz-type activators do not require the separate addition of other halide donors in order to achieve high cis-1,4-contents. In Nd-based catalyst-systems the dual role of RXA1 C1Z compounds is demonstrated by Watanabe and Masuda [364], These findings only hold true for Nd-based catalyst systems. For lanthanum-based catalyst systems Lee et al. found that the use of alkyl aluminum chlorides results in trans- 1,4-polymerization (93-94%) [371]. However, usually, in Nd catalysts the alkylating power of RxAlyClz is not sufficient at the applied amounts of RXA1 C1Z. Thus, an additional standard cocatalyst has to be added for the activation of the Nd precursor. 

Variations of the amount of cocatalyst which are usually expressed by the molar ratio W Nd have a significant influence on polymerization rates, molar masses, MMDs and on the microstructures of the resulting polymers. These aspects are addressed in the following sections with a special emphasis on ternary catalyst systems. For ternary systems it has to be emphasized, however, that in many reports the ratio Ai/ Nd only accounts for the amount of aluminum alkyl cocatalyst and not for other Al-sources such as alkyl aluminum halides. Variations of the Ai/ Nd-ratios are also used for defined control of molar mass. This aspect is addressed in separate sections (Sects. 2.2.8 and 4.5). 

A second method of production utilizes the Ziegler-Natta TiCl4 catalyst with liquid cocatalysts such as an alkyl aluminum halide. This is a reactive catalyst that must be prepared at the exclusion of air and water. The alkyl group of the co-catalyst coordinates with the Ti+3 site. The polymer grows by insertion of the ethylene into the double bond of the adsorbed polymer on another site. 

Elastomeric copolymers are made by either solution or suspension process using a vanadium based catalyst along with alkyl aluminum compound as cocatalyst. In the suspension process propylene is used as a diluent, whereas in the solution process hexane is used as diluent. Superior catalysts based on supported titanium compounds have further improved the suspension process in recent years. In the conventional suspension process, ethylene, propylene and catalysts are fed continuously to a stirred reactor at 20 °C and 12 kg cm total pressure. Diethylzinc is used to control molecular weight. 

Both homogeneous and heterogeneous Ziegler-Natta catalysts must be activated by a cocatalyst. Cocatalysts are alkyl aluminum compounds such as trimethyl aluminum (TMA) and triethyl aluminum (TEA). Cocatalysts are essential for polymerization with Ziegler-Natta, metallocene and late transition metal catalysts, as will be explained below. 

The transition-metal compoimd, usually a titanium(III) or titanium(IV) chloride, is transformed into the active catalytic species upon reaction with an aluminum alkyl cocatalyst. During this reaction the active metal becomes alkylated, and ethylene is inserted into the metal-alkyl bond. Often the active site is considered to be a bridging complex between the transition metal and the alkyl aluminum compoimd, in which one or two ligands are shared between the two metals. 

Metallocenes and Other Single-Site Cataiysts. One type of Ziegler catalyst, based on cyclopentadienyl titanium or zirconium halides (which provided only marginal activity using aluminum alkyls as cocatalysts) received an extreme enhancement in activity in the mid-1970s with the discovery of methylalumi-noxane (MAO) cocatalyst (68,69). Unlike traditional aluminum alkyl cocatalysts, MAO is far more capable of ionizing the transitional metal compound (57,70-73). 

The absence of functionality in hydrocarbon polymers limits applications where good adhesive properties, affinities for dyes, permeability, and compatibility with polar polymers are necessary. One of the limitations of conventional Ziegler-Natta catalysts is their intolerance of functional groups due to the high Lewis acidity of the transition metal component and the presence of Lewis acidic cocatalysts based on alkyl aluminums or aluminoxanes, as was anticipated in Section 2. 

We disclosed a few years ago that borohydride derivatives of the rare earths can advantageously be used as precatalysts for the polymerization of nonpolar monomers, in combination with metal-alkyl compounds as cocatalysts [12], Such catalysts were found to be very versatile as various monomers were successfully tested. Magnesium cocatalysts gave rise to controlled polymerizations, and the results were different depending on the precatalyst/cocatalyst ratio. Aluminum cocatalysts required the addition of a borate activator to afford polymers. Other catalytic combinations starting from phenate and MOF (metal organic framework) derivatives of the rare earths were also assessed and compared with the borohydride-based ones. 

Statistical copolymerization of ethylene and isoprene was achieved by using a borohydrido Cp (BH4)2Nd(THF)2] (Cp = C5Me5) half-lanthanidocene under polymer chain transfer conditions, and with lithium alkyl aluminum as cocatalyst (Al/Nd = 5). Polyisoprene-co-ethylene was received, with ethylene amount incorporated of circa 25 mol%, and the stereospecificity of isoprene enchainments was found to be around 96% translA- [38], It is noteworthy that, in turn, isoprene/hexene copolymerization did not succeed with the same precatalyst. This was attributed, after theoretical calculations, to a difference of reactivity by comparison with isoprene/ethylene mixtures. It is noteworthy that the absence of chain transfer conditions cannot be advanced to explain this result as we checked that isoprene/hexene copolymerization failed as well with a borohydrido lanthanidocene and excess BEM. 

According to our approach, we conducted in a first reaction step the copolymerization of PO with AGE, in the presence of catalytic systems consisting of an alkyl aluminum (e.g. triisobutylaluminum [ITBA]), controlled amounts of water and different organic compounds that act as cocatalysts, e.g. ethers, diols, phosphines, salieylie aeid derivatives, and organozine eompounds. Copolymers of type 1 having varying numbers of comonomer units in the ehain have been obtained (Seheme 1). 

Overview A large number of catalysts based on vanadium [35-37], titanium [38 0], zirconium [41], hafnium [42], lanthanides (in particular neodymium, samarium, and ytterbium) [43], cobalt [44, 45], niobium [46], chromium [47], nickel [48], and palladium [49] provide well-defined polyolefins. Many of these systems are able to meet the requirements for living polymerizations by suppressing P-hydride and P-methyl eliminations, as well as chain transfer to cocatalysts, such as alkyl aluminums or methylaluminoxane (MAO). Since MAO is usually obtained as a liquid solution with residual trimethylalumi-num, drying MAO to a white powder and removing residual trimethylaluminum can help minimize chain transfer to cocatlysts. 

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